Recently, personal computer and workstation manufacturers have introduced a large number of products having video displays that use display buffers with storage for 8 bits of gray scale or for 8 bits of each of the three primary colors. The popularity of 8-bit display buffers is understandable because of the universality of memory word lengths, bus widths and disk and network software that use an 8-bit byte (or some multiple thereof) as the basic unit of information. Furthermore, it has been well demonstrated that the human visual system cannot distinguish even the 256 levels of gray that can be coded in 8 bits. It seems likely that this defacto standardization on 8-bit displays and display buffers will continue and even become more pronounced with the near term introduction of increasingly compact, low cost workstations.
In contrast to this trend, many applications require video displays and display buffers with higher intensity resolution than 8 bits. In particular, images from film scanning and computed radiography have intensity resolutions from 10-12 bits and radiologists are generally unwilling to sacrifice the information present in these images to take advantage of the economies of 8-bit display systems. In video displays adapted specifically for those applications, the radiologists can adjust contrast controls by positioning an adjustable intensity window and through use of several settings of these controls utilize all of the intensity information present in the image even though less than 256 levels of gray can be seen for any one setting of these controls. In other words, when the radiologist is viewing a portion of the image which appears particularly dark, he can adjust the intensity window to lighten up this particular area to view the otherwise imperceptible detail therein due to the low intensity of that particular portion. Similarly, the radiologist can adjust the intensity window in the opposite direction to view the detail in other areas of the image which appear to be overexposed. Thus, although the human eye can discern differences of intensity at any one time of less than 256 levels of gray (8 binary bits), having the additional information present as provided by the four extra bits of intensity permits additional detail to be analyzed in the image.
While 12-bit video displays are available, they are more expensive than 8-bit displays as they utilize 12-bit architecture, and are not built in the same quantities as 8-bit displays. Additionally, they require 12-bit display buffers for storing digital data corresponding to the images. Obviously, 12-bit display buffers are more expensive than 8-bit display buffers and with the simultaneous use of both 8-bit and 12-bit display systems, one system's display buffers is incompatible with the other system's display which results in reduced ability to share information between systems.
To solve these and other problems in the prior art, the inventors herein have succeeded in designing and developing a high speed encoding circuit and high speed decoding circuit for transforming 12-bit pixel intensity data to an 8-bit, partially encoded, byte and also decoding the 8-bit byte into the 12-bit intensity data "on the fly" so as to permit an 8-bit display device to display in a flicker-free manner the pixels comprising a 1024.times.1024 frame with 12 available bits of intensity. The term "flicker free" is well understood to be that minimal display refresh rate required in order to render the flickering of the display caused by the periodic refreshing thereof to be imperceptible to the human eye. For most displays and applications, this rate has been found to be approximately 60 frames per second. Both the encoder and decoder are hardware based circuits, thereby avoiding the inherent delays required for those algorithms which are computationally complex or storage intensive and which ordinarly require the execution of computer instructions or other software based logic. The decoding circuit disclosed herein can be readily incorporated into a new video display device having an 8-bit architecture and the circuit can be easily enabled or disabled so that the display device can be used to display frame buffers storing 8 bit intensity data or frame buffers storing partially encoded 8-bit bytes representative of 12-bit intensity data. The encoder is particularly useful in converting 12-bit data and storing it in the 8-bit partially encoded format disclosed herein in 8-bit frame buffers for later display by a video display device. Although image sources are not presently available which produce 12-bit intensity images at sufficient rates of speed to be "on-line" with the video display device capable of producing images having 12-bit intensity, it is anticipated that such image sources will become available such that the encoder and decoder circuits and techniques disclosed herein may be used to view images at 12-bit intensity levels in real time with 8-bit architecture hardware.
In addition to its use in manufacturing new video displays, the present invention may also be utilized in retrofit kit form to convert existing 8-bit video displays for use in displaying images at 12-bit intensity levels. Thus, the present invention may be used to upgrade existing video displays and prevent their becoming obsolete with the implementation of the techniques disclosed herein.
In essence, the encoding and decoding techniques relied on herein for achieving compression of 12-bit data to 8-bits involves predicting the value of a pixel to be transmitted by calculating the difference between its intensity and its predecessor's intensity and transmitting only the differential obtained from this calculation. The differential is itself a 13-bit number which is divided into a first portion which is transmitted without encoding. In the preferred embodiment this portion is 5 bits wide. The second portion of the differential data is compactly encoded so that the average number of bits transmitted per pixel is comparable to the average entropy per pixel. The goal is to transmit as much information as possible without encoding so as to minimize the encoding and decoding task to thus simplify it and achieve high speed data transmission rates.
While the invention is disclosed generally for use with images of various types, it has been found to work best with images generated through over sampling which are characterized by pixels having a relatively high degree of correlation. Thus, differential values between adjacent pixels are minimal such that encoding the differential value can be efficiently done with the coding techniques disclosed herein.
While the principal advantages and features of the present invention have been described above, a clearer understanding of the invention and its purposes may be attained by referring to the drawings and description of the preferred embodiment which follow.